The team notes, however, that the hollows are much smaller than known volcanic pits, and the holes appear in places on Mercury that aren't likely to have experienced volcanic activity.

What's more, the hollows look distinctly fresh, because they haven't been reshaped by later impact events.

The shapes of Mercury's hollows bear some similarities to holes seen in the polar ice caps on Mars, in regions known to astronomers as Swiss cheese terrain, Blewett said.

The Martian depressions form as carbon-dioxide ice sublimates—turns directly from a solid to a gas—during seasonal temperature changes, hinting that some type of sublimation may be happening on Mercury.

"But on Mercury it's happening in solid rock, not in ice, so it's sort of a unique expression of geological processes that happen elsewhere, but maybe not as vigorously," he said.

Since the tiny planet has no atmosphere, these particles can hit the surface directly, vaporizing volatile minerals. Or the close sun's intense heat could "boil" the minerals away.

"Say there was a sulfide mineral that gets zapped by a solar wind particle," Blewett said. "That sulfur could be lost, and the rock these minerals are in is basically undermined and would crumble away."

Mercury Not a "Burned-Out Cinder"

Researchers aren't yet sure what Mercury's rocks are made of, although one goal of MESSENGER's mission is to map the planet's surface composition.

In another paper also released this week, for instance, scientists using MESSENGER's x-ray spectrometer found that Mercury's surface has much more sulfur than that of any other rocky planet in the solar system.